In many optical applications, small-scale components are used to manipulate light beams. For example, lenses provide beam focusing or beam collimation. As another example, reflective surfaces are used to redirect light, such as in the coupling of a light source to an optical fiber. The optical axis of the light source may be at a right angle to the optical axis of the fiber. A 45 degree mirror redirects the light from the source to the optical fiber. As used herein, a “mirror” is synonymous with a “reflector.”
A great amount of effort has been placed on maximizing the smoothness of the mirror surface. Any surface roughness or curvature has a potential of introducing distortions in the light signal. One known approach to fabricating a small-scale mirror is to use anisotropic etching of silicon. Certain wet etchants, such as potassium hydroxide (KOH), will etch primarily in the direction of the crystal plane. The section entitled “Description of the Related Art” in U.S. Pat. No. 6,417,107 to Sekimura describes one known etching technique for forming a 45 degree mirror. Firstly, a silicon ingot may be sliced at an angle to obtain a <100> silicon wafer which is 9.74 degrees off-axis. Without the oblique cut, the wet etchant would etch an angle of 54.74 degrees. However, the off-axis silicon substrate is etched at an angle of precisely 45 degrees (54.74−9.74). The etching angle is determined by the orientation of the <111> crystalline plane, which typically has a very slow etch rate. This property enables the <111> crystalline plane to be used as an etch stop. Thus, in the 45 degree mirror, the reflective surface is along the <111> crystalline plane.
There are a number of factors that affect the planarity (i.e., smoothness) of the resulting <111> crystalline plane. Techniques have been introduced to increase the planarity. It is a common practice to add a surfactant into the etchant in order to improve surface smoothness. For example, isopropyl alcohol may be introduced into the KOH. As another, arsenic salt has been added to passivate and smooth the etched surface. The Sekimura patent describes using an etchant of KOH or tetramethylammonium hydroxide (TMAH) with a non-ion type surface active agent, such as polyoxyethylene alkyl phenyl ether. It is also known to introduce an impurity into the silicon crystal itself in order to reduce roughness on the surface.
Annealing a rough silicon surface in a reduced pressure hydrogen atmosphere can improve the smoothness of a silicon substrate. It is possible that the planarity of a mirror surface may be improved after it is formed, if the anneal is applied. During the etching process, planarity can be improved by reducing or eliminating the occurrence of bubbling on the silicon surface. Hence, either oxygen or hydrogen gas can be bubbled into the etching bath.
Other concerns in the etching of silicon to form a 45 degree mirror relate to the tediousness and the repeatability of the process. Photo masks are typically used. Accurate alignment of the photo mask to the crystal axis normally includes a two-step etching approach. The first etching step reveals the true crystal orientation. Then, the second etch requires precisely aligning the etching mask to the revealed crystal orientation. The process is both tedious and subjective.